This invention relates to the measurement of ultrasonic subsurface waves velocities. Times of flight or velocities of such waves are used to evaluate elastic properties, texture or stresses in materials under test. Subsurface waves are desirable when the access is limited to only one surface of the workpiece or only surface layer of the workpiece material is to be evaluated.
For these measurements various sets of ultrasonic probes are used. The basic set consisting of one transmitting and one receiving probeheads is described in U.S. Pat. No. 4,398,421 to White. Both probes are equipped with piezoelectric crystals and plastic wedges and are acoustically coupled to the workpiece surface by means of a coupling medium. The velocity of subsurface wave is determined by measuring the time between the transmission by the first crystal and reception by the second crystal and factoring the measured time into the predetermined distance.
Another method to determine the velocity of surface wave is described in U. S. Pat. No. 4,372,163 to Tittmann. A pair of probes, one transmitting and one receiving surface wave, is used. Two times between the transmitting and receiving crystals are measured for two various distances between probes. The velocity of waves is determined by factoring the difference in measured times into the difference in distances between probes.
The so-called "differential set of probes", consisting of one transmitting probe and two receiving probes, arranged along one line, is described in Polish Patent 13 8042 to Borkowski et al. and by E. Schneider et al., Automatisierte Bestimmungoberflaschennaher Spannungszustande in Walzen mittels Ultraschallverfahren, Mat. Konf. DGZfP, pp. 419-425 (Kiel 1989). Two times of subsurface wave are measured: the first one between transmission by the transmitting crystal and reception by the first receiving crystal, and the second one between the transmitting crystal and the reception by the second receiving crystal. The velocity of wave is determined by factoring the difference of measured times into the distance between receiving probes.
The weakness of the known sets of ultrasonic probes is that their performances can be easily adversely affected by the temperature or the roughness of surface of the workpiece under test. Ultrasonic subsurface or surface wave transmitted by the crystal of the first probes in order to be detected by the crystal of the second probe, has to travel all the way long through the wedge, coupling medium layer and workpiece surface layer and also through the second layer of coupling medium and the second wedge. The time of flight of ultrasonic wave in the plastic wedge is strongly dependent on the wedge temperature.
Time of propagation of such waves between crystals is also dependent on the thicknesses of the layers of the coupling medium. These thicknesses are functions of the workpiece roughness which can vary over the workpiece surface.
Assuming the temperature of the wedges in both receiving probes of "differential set" are the same, due to the fact that the set consists of one transmitting and two receiving probes, the influence of temperature on measured time is eliminated. Also in the method described in U.S. Pat. No. 4,372,163 the influence of temperature is eliminated. However, for all known sets of probes, the determined velocity of subsurface or surface waves still depends on irregularities of the workpiece surface roughness. Due to these irregularities the thickness of the layer of the coupling medium between the wedge of each probe and the workpiece surface can be different. To eliminate the influence of surface roughness on results obtained with known sets of probes it is necessary to prepare the workpiece surface.
It is an object of this invention to provide a novel and improved method for measuring the velocity of ultrasonic subsurface waves in a workpiece with rough surfaces, without time-consuming surface preparation. Railroad rails and other rolled products provide a good example of workpieces with rough surfaces, in which velocities of subsurface waves are to be measured in order to determine residual stresses or texture. The features of our invention will be understood from the study of the following description and drawings showing preferred embodiment of the invention.